The present invention relates to the art of fluid containment. It finds particular application in conjunction with tanks utilized in effluent disposal systems, such as septic tank effluent pump systems, variable grade sewer systems, and small diameter gravity sewer systems. However, it is to be appreciated that tanks in accordance with the invention are also suitable for storing fluids and fluid-type materials both above and below ground.
Sewage and septic systems have traditionally been constructed to receive fluid by gravity flow from a residence and discharge the fluid by gravity flow to an absorption field. The absorption field having excess fluid discharge capacity and being accessible to the in-flow of high ground water, no particular effort was commonly made to seal the tank or the connecting lines against ground water. During periods of high ground water levels, this leakage enabled additional fluid to flow into the tank to compensate for the increased hydraulic pressure of surrounding ground water and water saturated soil.
For better environmental control, septic systems are now being installed in which the tank connects with a central sewage processing station rather than a individual absorption field. Central sewage processing facilities are commonly designed to treat sewage at a preselected number of gallons per hour. The determination is made based on the number of homes connected with the central processing system and the average daily output of the homes, typically considered to be about 150 gallons per day. During heavy rains, ground water leakage into the prior art septic tanks and sewage systems can increase the flow rate to the central sewage processing facility by a factor of 10 or more. For example, a 1/32 inch wide gap around a 24-inch diameter riser under one inch of head would infiltrate 600 gallons per hour into the tank, i.e. four times the average daily discharge. To prevent the overloading of the treatment facilities, to save the cost of building greatly oversized central processing facilities, and to prevent the discharge of raw sewage during heavy rains, the septic tank and associated systems must be sealed against ground water.
Sealing the tank against ground water increases the tendency for in-ground tanks to collapse under the increased hydraulic pressure differential between high ground water and the tank interior. Collapsing is particularly prevalent when the ground water level exceeds the level of effluent within the sealed tank. The high ground water might collapse a tank directly. However, the high water more commonly distorts the carefully crafted structural shape of plastic tanks, cancelling the strength provided by the undistorted structure. The distortion related loss of strength leads to further distortion until the tank collapses or fails. Such distortion often occurs adjacent the upper portion of the tank which is above the internal effluent level.
Another common point of failure is at the flat bottom of the tank. Excessive hydrostatic pressure arches or distorts the bottom wall inward, drawing in the side walls. The distortion to the side wall corrugations reduces the structural strength that they provide, allowing further distortion and failure.
The hydraulic pressure differential has further been increased in systems in which fluid is pumped from the interior of the tank to the central processing station. This pumping tends to reduce the pressure within the sealed tank.
The present invention contemplates a new and improved tank which overcomes the above referenced problems and others.